1. Field of the Invention
The present invention relates to a discharge lamp lighting device used for a head lamp of a car, an illuminating lamp in indoor/outdoor facilities, a warehouse and a factory, a streetlamp, etc.
2. Description of the Related Art
Among discharge lamps, high-intensity discharge lamps (HID bulbs) such as a metal halide bulb, a high-pressure sodium bulb, and a mercury bulb have been heretofore used as illuminating lamps in indoor/outdoor facilities, warehouses and factories, streetlamps, etc. because the high-intensity discharge lamps have the advantages of wide light flux, high lamp efficiency, long life, etc. Particularly in recent years, the high-intensity discharge lamps have been used also as head lamps for vehicles such as cars.
To switch on the discharge lamp of this type, starting pulses of a high voltage need to be superposed on a predetermined voltage applied to the bulb at the time of stating the bulb. Therefore, a DC/DC converter and an inverter are provided for lighting the discharge lamp stably while an igniter (starter) is provided for generating starting high-voltage pulses.
FIG. 5 is a circuit diagram of a related-art HID lamp lighting device described in JP-A-2001-143890, etc. FIG. 6 is a time chart for explaining the operation of the related-art HID lamp lighting device.
In FIG. 5, the reference numeral 1 designates a DC source such as a car battery; 2, a discharge lamp lighting device; 3, a DC/DC converter; 4, an inverter made of an H bridge or the like; 5, an igniter; and 6, a discharge lamp. Voltages Va, Vc and Vd shown in FIG. 6 indicate voltages at points a, c and d shown in FIG. 5, respectively.
The polarity of the voltage Va output from the DC/DC converter 3 supplied with a voltage input from the DC source 1 is inverted periodically by the inverter 4, so that rectangular AC voltages represented by the voltages Vc and Vd are applied to the discharge lamp 6. The voltages Vc and Vd applied to the discharge lamp are formed as negative voltages so that the average potentials become negative to prevent devitrification of metal enclosed in the discharge lamp bulb. In the related-art example, all electric power supplied to the discharge lamp is electric power output from the DC/DC converter 3.
FIG. 7 is a circuit diagram for explaining another related-art example described in JP-A-2002-159172.
In FIG. 7, the reference numeral 1 designates a DC source such as a car battery; 3, a DC/DC converter; 7, a transformer; 8, a switching device; 9, a rectifier; 10, a capacitor; and 11, a load circuit (discharge lamp).
In the related-art example, the capacitor 10 has no function but a smoothing function. Accordingly, the capacitor 10 is incapable of supplying a DC current to the load circuit steadily, so that a current I10 does not flow at all. Accordingly, the DC/DC converter 3 provides all the output current I0 from a current It. For this reason, the voltage V1 of the DC source 1 is not superposed on the voltage V10 of the capacitor 10, so that only the voltage generated in the transformer 7 is applied to the load circuit.
In the related-art discharge lamp lighting device, all electric power supplied to the discharge lamp is electric power output from the DC/DC converter. Accordingly, the output capacity of the DC/DC converter needs to be designed according to electric power required for switching on the discharge lamp. Particularly in a car discharge lamp lighting device requiring high electric power at the initial stage of lighting, there was a problem that it was impossible to reduce the size of the DC/DC converter, especially it was impossible to reduce the size of the transformer in the DC/DC converter.